Conventionally, installed optical fibre networks have been tested to detect a fault such as a break in the network by connecting a dedicated optical time domain reflectometer (OTDR) to the network. The OTDR typically comprises a high power optical source which transmits a pulse onto the network. Light back-scattered from faults in the network is detected and correlated with the transmitted pulse to determine the location of the fault giving rise to the back-scattered signal.
As described in the present applicant's earlier international application WO 91/02959 the use of dedicated OTDR equipment has significant disadvantages. In particular it requires the interruption of the service normally carried on the network and so is normally used only after a failure occurs and is not suitable for continual monitoring of the network. The system described and claimed in the above cited application proposes the transmission of an appropriately coded OTDR signal as a part of each downstream data frame sent onto the network. In the preferred example the test pulse sequence is a Barker or Golay code transmitted in a header portion of a multiplexed data frame. Although this arrangement does make it possible to monitor the network continually there is then the disadvantage of a significant transmission overhead associated with the dedicated OTDR codes and a subsequent loss of bandwidth which might otherwise be used for data or for other system control functions.